Caught in the Act: A Case Study on Microscopic Scale Physicochemical Effects of Fossilization on Stable Isotopic Composition of Bone

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Geochemical zoning in a partially fossilized Pleistocene artiodactyl metapodial was contrasted with a fully fossilized Cretaceous ceratopsian femur as distinct stages along the fossilization process to investigate the physicochemical effects of fossilization and fossilization mechanisms. Bone fragments were analyzed via laser ablation ICP-MS, FT-IR, and stable isotope microanalysis to determine zoning patterns in trace elements, relative carbonate content and collagen contents,and stable isotopes of both the CO3 and PO4 components (δ13CCO3, δ18OCO3, and δ18OPO4). The Pleistocene sample shows a pronounced step in U concentration from high values (up to 90 ppm) within 1 mm of the outer bone surface and inner medullary cavity to ppb-level concentrations typical of in-vivo bone in the interior (inner ~6 mm). High U broadly coincides with reduced collagen, higher δ13CCO3, and lower δ18OCO3. Rare earth elements (REE), however, show typical exponential decreases from bone edges inward, and δ18OPO4 shows no clear trend. The Cretaceous bone shows high REE and U contents throughout, a complete absence of collagen, and relatively uniform isotope compositions. The Pleistocene data, especially U zoning, implicate a diffusion-reaction (DR) fossilization mechanism, in which collagen degradation facilitates a recrystallization front that propagates as a front towards the bone interior, reducing OH-site CO3 content, and shifting δ13CCO318OCO3,and δ18OPO4. Disequilibrium O-isotope partitioning between the CO3 and PO4 components in the interior of the Pleistocene bone suggests that both the CO3 and PO4 components have exchanged with diagenetic waters, likely through abiotic and microbially-induced exchange, respectively. Strong partitioning between apatite and soil water, coupled with uptake of REE in apatite from surrounding sediment matrix during fossilization, leads to exponential decreases in concentrations, which are passively recorded in apatite in the bone interior. The Cretaceous data do not implicate a unique fossilization mechanism but instead reflect the consequences of protracted fossilization and the final state that the Pleistocene bone might eventually have reached had the bone not been excavated. That is, the Cretaceous bone fossilization history resulted in no collagen, reduced or no OH-site CO3, and more uniform isotopic compositions. The multi-analytical methods employed in this study may benefit studies of diagenetic alteration of other apatitic tissues, e.g., tooth enamel.